This invention relates to improved methods of detecting and imaging an object or objects contained in a medium which object or objects have contrasting electrical conductivity and/or specific impedance compared to the medium.
It was reported in 1926 by H. Fricke and S. Morse in the article xe2x80x9cThe electric capacity of tumours of the breastxe2x80x9d, (1926) J. Cancer Res. 16, pp. 310-376 that the electrical properties of breast tumors differ significantly from healthy tissue. Until now it has not been possible to use these properties to detect breast tumors in a manner useful in a clinical setting. However, some laboratory and imaging techniques have evolved.
Electrical Impedance Tomography (EIT) is an imaging methodology that is based upon electrical conductivity or impedance contrasts within the human body. EIT has been the subject of considerable attention recently but, generally, methods used for image recovery have yielded only low-resolution results.
U.S. Pat. No. 4,539,640, issued Sep. 3, 1995, to inventors Bradley Fry and Alvin Wexler (referred to below as the Wexler et al patent), and the article by A. Wexler, B. Fry and M. R. Neuman, entitled xe2x80x9cImpedance-Computed Tomography: Algorithm And Systemxe2x80x9d. Applied Optics, Vol. 24, No. 23, Dec. 1, 1985-pp. 3985-3992, describe a method and embodiment of a system that solved electromagnetic field equations that govern current flow in a conductive medium, and concurrently extracted an image of the interior of the medium based on the electric current conductivity (and, more generally, specific impedance) distribution in the medium. This provided a methodology for the correct mathematical solution of the inverse (imaging) problem and construction of electronic equipment for this purpose. The method also provided for the accommodation of a great number of pixels through the use of spa-matrix techniques. U.S. Pat. No. 4,539,640 is incorporated herein by reference.
This methodology embodies a number of technological features, e.g. uses a well-conditioned, least-squares method, uses true three-dimensional field solving for images and conductivity values and for identification of characteristic pathologies, is applicable to complex impedance as well as to purely conductive imaging, it allows for application of known CT-based reconstruction methodologies and image processing operations between iterations, only simple contact of electrodes to the skin or to geophysical or other surfaces is sufficient to provide for contact and spreading resistance, it results in a sparse-matrix formulation for high-definition imaging, parallel data acquisition may be performed through frequency multiplexing for speed, and parallel image reconstruction may conveniently be accommodated.
EIT uncovers objects within a host medium by solving for resistivity (or, more generally, specific impedance) distributions within the body. Various techniques have previously been used that treat the flow of applied electrical currents as though they behave in a manner similar to X-ray beams. With this assumption, algebraic reconstruction techniques (ART) originally described by Gordon, Bender and Herman in the article xe2x80x9cAlgebraic Reconstruction Techniques (ART) For Three Dimensional Electron Microscopy and X-Ray Photographyxe2x80x9d, (1970), J. Theor. Biol. 29, pp 471-481, have been employed by others to uncover a crude approximation of the EIT image.
ART finds wide and accurate use in applications of computed tomography other than EIT. Because electrical currents between any two electrodes flow throughout the body and do not follow ray-like paths, a straightforward application of ART is inappropriate. Therefore, as confirmed by R. H. T. Bates, G. C. McKinnon and A. Seager in the article xe2x80x9cA Limitation On Systems For Imaging Electrical Conductivity Distributionsxe2x80x9d, (1980), IEEE Biomed Eng. BME-27, pp. 418, a comprehensive field-solving approach is needed as part of the EIT imaging process.
A considerable body of EIT work has been done at the University of Sheffield in the U.K. Smith, Freeston and Brown describe their Applied Potential Tomography (APT) system which uses a weighted back-projection technique, in xe2x80x9cA Real-Time Electrical Impedance Tomography System For Clinical Usexe2x80x94Design And Preliminary Resultsxe2x80x9d, (1995), IEEE Trans. Biomed. Eng. BME-42, pp. 133-140.
Guardo et al describe a back-projection reconstruction method which could detect a 3 ml plastic sphere at the centre of a torso-sized cylinder of saline, in xe2x80x9cAn Experimental Study In Electrical Impedance Tomography Using Backprojection Reconstructionxe2x80x9d, (1991), IEEE Trans. Biomed. Eng. 38 (7), pp. 617-627. This translates to approximately a 1.5 cm sphere in a 50 cm cylinder.
Shahidi, Guardo and Savard in xe2x80x9cElectrical Impedance Tomography: Computational Analysis Based On Finite Element Models Of The Human Thorax With Cylindrical And Realistic Geometriesxe2x80x9d, (1995), Annals Biomed. Eng. 23 (1), pp. 61-69, report that three-dimensional finite element method simulation results show that xe2x80x9ca 10 ml edema region with a conductivity equal to that of blood can be detected at a 40 dB signal-to-noise ratio (SNR)xe2x80x9d, and further: xe2x80x9cDetection of a smaller volume, in the order of 2 ml, should be possible by improving either the instrumentation to achieve 60 dB SNR or the performance of the reconstruction methodsxe2x80x9d. These results, scaled to the size of the breast, indicate that even small breast tumors (less than 4 mm in diameter) should detectably alter surface potentials.
It should be noted that detection is not image reconstruction but it is a necessary precondition. Guardo""s research team has demonstrated that a practical, measurable signal is available for use in EIT.
Henderson and Webster presented a means for displaying isoadmittance contours of the chest, Tasto and Schomberg described an impedance imaging technique that considers curved current flux tubes and uses back-projection techniques, Lytle and Dines report on the use of impedance techniques for geophysics applications and Price further discusses medical applications and techniques (Henderson, R. and Webster, J. (1978) xe2x80x9cAn Impedance Camera For Spatially Specific Measurements Of The Thoraxxe2x80x9d, IEEE Trans. Biomech. Eng. BME-25, pp. 250; Tasto, M. and Schomberg, H. (1981) xe2x80x9cMethod Of And Device for Determining Internal Body Structurexe2x80x9d, Washington, D.C., U.S. Pat. No. 4,263,920; Lytle, R. J. and Dines, K. A. (1978) xe2x80x9cAn Impedance Camera: A System for Determining the Spatial Variation of Electrical Conductivityxe2x80x9d, (1978) Livermore, Calif.: Lawrence Livermore Laboratory Report UCRL-52413; and Price, L. R. (1979) xe2x80x9cElectrical Impedance Computed Tomography (ICT): New Imaging Techniquexe2x80x9d, IEEE Trans. Nucl. Sci. NS-26, 2736.
An alternative method as described in the aforenoted U.S. Pat. No. 4,539,640, involves the application of currents to the body and successive measurement of surface potentials. This image recovery method involves the solution of the Poisson/Laplace equation while employing sparse-matrix techniques.
Dijkstra, A. M., B. H. Brown, A. D. Leathard, N. D. Harris, D. C. Barber, and D. L. Edbrooke xe2x80x9cReview: Clinical Applications Of Electrical Impedance Tomographyxe2x80x9d, (1993), J. Med. Eng. Technol. 17 (3), pp. 89-98 discuss clinical applications of EIT. They review the conductivity of tissues at around 50 kHz, and show the large contrasts that exist. In their view, xe2x80x9c. . . the major disadvantage is the poor spatial resolution which is only about 10% of the diameter of the body. It seems likely that this may be improved to 5% (1 cm in a body of 20 cm diameter) and at this point it begins to be similar to that offered by a gamma camera. We should therefore regard the technique as a monitor of body function and not as an anatomical imaging method.xe2x80x9d
We believe that this conclusion is pessimistic, as the present invention provides a practical EIT method and apparatus with high resolution that can be used in a clinical setting.
In one aspect of the present invention, we have developed methods to vastly improve the quality of edge detection at interfaces between surrounding (e.g. breast) tissues and small included objects (e.g. tumors), to accurately and rapidly image small objects and to substantially increase the speed of image extraction. It is believed that this provides a practical method and means that can be used for detection of breast tumors in a clinical setting.
It is known that breast tumor metastasis rapidly increases with tumor size. It has been shown that if all breast tumors are detected and removed by the time they reach 4 mm diameter, total metastasis would be 0.6%, indicating a 99.4% cure rate. We therefore also believe that an effective and relatively inexpensive EIT machine built in accordance with the principles of the present invention, used as a screening tool to find such tumors, could make breast cancer a highly curable disease.
It is believed that by the use of the present invention, imaging of objects that are 5% of body diameter in size are achievable now and it is expected that the method can be used to image objects that are 2.5% of body diameter or smaller. This translates to breast tumors of 5 mm and 2.5 mm respectively. We believe that our new method will provide sufficient spatial resolution to detect tumors smaller than those typically found via mammography.
The principles of the present invention can also be applied to other imaging uses such as subterranean imaging, for the purposes of detection of contamination of groundwater, for location of landmines and other explosives (including those that are plastic encased), etc.
It is expected that use of the present improved methods for the imaging of objects located within bodies in EIT scanners for medical applications will be preferred because they are noninvasive and can be produced relatively inexpensively, as compared to X-ray CT (CAT scan) and MRI (magnetic resonance imaging), and may be operated by trained technicians. For geophysical applications, EIT scanners using the principles of the present invention provide an alternative to test boring and will be able to identify landfill site leachates prior to contamination of groundwater resources. It is believed that they can also be used to image plastic-coated land mines, which otherwise are very hard to find.
In another aspect of the present invention, a method of imaging an object or objects contained in a medium, having a conductivity which is different from the conductivity of the medium, comprises the steps of applying current to the medium at various locations at a surface of the medium, extracting current at various locations, detecting voltages due to currents which have passed through the medium from the surface of the medium at various other locations, successively determining the location and shape of the object with increasing accuracy by processing values of the detected voltages, and determining a region of the medium in which the objects are located from values of the detected voltages.
As a consequence of processing for current values, the method can process for impedance, either resistive or complex.